I. Field
This invention relates to the field of communications systems and, in particular, to a method for controlling the transmission power level of multiple data streams sent from one or several base stations to a mobile station in a mobile radio telecommunication system.
II. Prior Art
In a mobile telephone communication system, one or several base stations transmit information, such as voice information, or data, or both to a mobile station. Each base station supports one or several sectors. For example in EIA/TIA-95-A CDMA systems it is common that each base station supports three individual sectors, with each sector transmitting different information. Voice and data transmissions from a base station to one or more mobile stations typically occur on a forward link traffic channel. A mobile station receives the information from the forward link traffic channel, decodes the information, and determines a frame error rate associated with the decoded information. The frame error rate of the decoded information can be adversely affected by, for example, fading conditions in the forward link channel. Furthermore a traffic channel can be transmitted from several base stations or several sectors of the same base station. The mobile station will then combine the signals from the different sectors for improved decoding, in a process that is often referred to in the prior art as soft-handoff. The set of base station sectors transmitting the same data signal is usually named an “active set”. It will be understood by those skilled by the art that the term soft handoff refers to soft handoff between different base stations as well as soft handoff between different sectors of the same base station.
In some mobile radio communication systems such as, for example, mobile radio systems that use code division multiple access (CDMA) modulation, the frame error rate at the mobile station is used to control the transmit power level sent to the mobile on the forward link traffic signal. For example, in such systems a desired ratio of signal to noise powers is derived from the desired frame error rate. An estimate of the actual signal to noise ratio received by the mobile is then used to generate a stream of power control commands that is sent from the mobile station back to the base stations in the active set. Each power control command in the stream causes the base station to either increase (by, for example, 1 dB), decrease (by, for example, 1 dB) or hold constant the transmit power sent to the mobile station on the forward link traffic channel.
Using such a power control system allows the mobile station to cause the base station to increase the transmit power to compensate for conditions such as a fade. Likewise, the power control system permits the base station to save power when the channel conditions are more favorable and a predetermined error rate can be maintained using a lower transmit power.
In modem mobile telephone communication systems, several data streams (e.g., fax transmissions, internet transmissions, voice calls etc.) can be transmitted to a mobile station concurrently. In systems such as CDMA systems, the transmission of such data streams can occur on the same forward link traffic channel (i.e., frequency channel). In such cases, each data stream (e.g., voice, fax, internet, etc.) transmitted from a particular base station to the mobile station on a given forward link is modulated using a different spreading code often called a Walsh code that permits each data stream to be separately demodulated at the mobile station. Different base stations can transmit on the forward link with the same spreading code when they utilize a different scrambling code (often called a PN code).
Where multiple data streams are transmitted from a one or several base stations to a mobile station on one or several forward links, the transmit power level of each of the data streams should be controlled as described above. However, sending a separate stream of power control commands on the reverse link from the mobile station back to each base station in order to control the transmit power of each data stream results in a substantial increase in system overhead.
Thus, it would be desirable to provide a system for forward link power control that minimized the overhead required to send power control commands from the mobile station back to a base station in cases where the base station is transmitting multiple data streams to the mobile station.